when he's standing still, keep charging him in the direction of the lava. when he's near the edge, charge him into the lava. Once he's in the lava and sheila's stomping him even more into the lava, charge to the other side of the arena thing, then when he begins to roll towards you charge around the outside of the circle until he stops. then repeat until he's dead.
--- "This is the most musically inclined situation I've ever found myself in and I've played guitar hero!"